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IndustryArena Forum > WoodWorking Machines > DIY CNC Router Table Machines > Any thought on my Z axis Design (first build) Steel vs Aluminium Plate?
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  1. #1

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    Any thought on my Z axis Design (first build) Steel vs Aluminium Plate?

    Hello, here is my Z axis design,

    I'm thought it would be better to use ground steel plate so I can reduce the distance the spindle extends from the gantry. I plan to use 8mm ground steel plate with some thin side plate to add stiffness. I haven't built a cnc before though so have doubts if ground steel plate will be flat enough compared to aluminium tooling plate. I have done a bit of hand scraping in the past and don't mind spending time if the steel isn't good enough.

    The supplier on one website states their ground steel plate is under 1mm per 1 meter, my Z plate is about 30cm long so 0.3mm would be the worst if I got a poor piece.
    Maybe i'm over estimating the importance of saving around 12mm extension from the y gantry if i did use say 20mm aluminum. I intend to use the cnc for hardwood, perhaps a bit of nonferrous metal too. I'd like to be able to machine things 8cm high at most.

    Any thoughts?
    Thanks

  2. #2
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    Re: Any thought on my Z axis Design (first build) Steel vs Aluminium Plate?

    Hi Thunza - 8mm is a bit thin for a Z plate by itself especially if you want to cut aluminium. Adding the flanges is a good strategy but yours need to be much deeper. 25mm thick steel is common and 30mm aluminium is also for a flat Z plate. Make the flanges much much deeper and it will work well. The images attached show one of my machines. The tool plate is equivalent to about 60mm thick steel by having the deep flanges... Its 3mm steel so the middle is 6mm thick the flanges are about 40mm each side. The two parts are epoxied together to create one part. Peter

  3. #3
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    Re: Any thought on my Z axis Design (first build) Steel vs Aluminium Plate?

    Hi,
    I agree with peteeng, 8mm is a joke, 20mm min and 32mm preference. Steel is just so much stiffer than aluminum, and cheaper too.
    Get some 32mm plate plasma or laser cut then take it to a tool maker and have them put it on the surface grinder.....easy. I got three pieces of
    350mm x350mm x 20mm surface ground for $50, dropped it off at lunchtime, and picked it up that evening....easy, flat and parallel to better than 0.01mm.

    Craig

  4. #4

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    Re: Any thought on my Z axis Design (first build) Steel vs Aluminium Plate?

    Thanks a lot for your input.

    I imagined 8mm steel is equal to 24mm aluminium if it's 3 times stiffer. I guess the extra distance between the aluminium faces mean it might not work quite as simply as that.

    Peeteng, its interesting you were able to go to just 6mm with the large flanges. Did you use that machine for just wood?


    So deeper flanges and thicker plate if I can't increase the flanges height. Hopefully it won't get too heavy if I were the increase plate thickness, I bought a set of 4nm Nema 23 motors to power the cnc. I suppose the plate itself isn't as large as some cnc's are, it will be 30 x 14cm if you don't count the little end section where the motor mounts.

  5. #5
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    Re: Any thought on my Z axis Design (first build) Steel vs Aluminium Plate?

    Hi Thunza - 1) no 8mm is not equal to 24mm as its 3x stiffer material see calc attached. The rigidity of a beam is equal to its material stiffness x its geometric inertia. Use 5mm pitch ballscrew for Z and it will lift 461kg so don't worry about Z weight. 2) Yes the I section Z plate easily cuts aluminium. Its the stiffest plate I have made on any of my machines. Would not go any "thinner" into the future 3) 24mm thick AL is equivalent to 17mm thick steel and its lighter. Peter

  6. #6
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    Re: Any thought on my Z axis Design (first build) Steel vs Aluminium Plate?

    Hi,

    I imagined 8mm steel is equal to 24mm aluminium if it's 3 times stiffer. I guess the extra distance between the aluminium faces mean it might not work quite as simply as that.
    No, that is not correct. It might surprise you that I calculated, or rather used Fusion's FEA tool to calculate, the equivalent thickness of steel and granite. Granite has about the same
    stiffness as aluminum, so the comparison will be close. A 32mm thick steel section has the same stiffness as a 50mm thick granite section despite steel being three times stiffer than
    granite.

    I would expect, say 16mm steel, to be approximately equal to 24mm aluminum.

    If you do the momentum equation for your axis its highly probable that the mass of the solid plate is likely to be less significant than the rotating ballscrew. Again
    hard to believe but the physics dictate that the momentum is often dominated by the ballscrew.

    Craig

  7. #7

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    Re: Any thought on my Z axis Design (first build) Steel vs Aluminium Plate?

    I'm surprised granite is only about as stiff as aluminium given granite isn't at all ductile.

    "Would not go any "thinner" into the future 3) 24mm thick AL is equivalent to 17mm thick steel and its lighter. Peter"

    I felt this might be true but didn't realize just how much of an effect the geometry has, even though they're both just plates. I'm glad I asked before going ahead with my plan.

    Thanks to both of you. I feel I can continue the design and buy parts now.

  8. #8

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    Re: Any thought on my Z axis Design (first build) Steel vs Aluminium Plate?

    I presume I could place the flanges to they come out instead of inwards like in this drawing? I would add a front brace too as the spindle carriage wont go past the end of the plate.

    The only issue might be it could make things fiddly with tramming and so on
    Attached Thumbnails Attached Thumbnails 3.jpg  

  9. #9
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    Re: Any thought on my Z axis Design (first build) Steel vs Aluminium Plate?

    Hi,
    I think you are making the part more complicated than it need be. If you used a thick solid plate, be it steel or aluminum would be by far and away the cheapest and
    easiest solution. Flanges add manufacturing complexity.

    Give us the details of the Z axis, namely the ballscrew diameter, ballscrew pitch, ballscrew length and then we can calculate the momentum equation which will likely show that the
    increased mass of the solid plate is insignificant.

    Just as an example: my new build mill has cast iron beds of 115kg each, but for the calculation I allowed the axis mass of 150kg. The ballcresws are 32mm diameter, 5mm pitch and 700mm long. The servo
    has a first moment of inertia of 1.1 x 10-4 kg.m2. The momentum calculation shows that 80% of the momentum is in the rotating ballscrew alone, 12.5% in the armature of the servo
    and the remaining 7.5% in the linear axis mass. So despite the axis weighing 150kg its still a minor contributor to the momentum. Thus if I decided to add a whole bunch of mass to the axis, say a trunnion table
    and fifth axis, the extra mass is STILL a minor to modest contributor to the overall momentum.

    Give us the details of the axis and we'll see just how important, or not, the mass is.

    Craig

  10. #10

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    Quote Originally Posted by joeavaerage View Post
    Hi,
    I think you are making the part more complicated than it need be. If you used a thick solid plate, be it steel or aluminum would be by far and away the cheapest and
    easiest solution. Flanges add manufacturing complexity.

    Give us the details of the Z axis, namely the ballscrew diameter, ballscrew pitch, ballscrew length and then we can calculate the momentum equation which will likely show that the
    increased mass of the solid plate is insignificant.

    Just as an example: my new build mill has cast iron beds of 115kg each, but for the calculation I allowed the axis mass of 150kg. The ballcresws are 32mm diameter, 5mm pitch and 700mm long. The servo
    has a first moment of inertia of 1.1 x 10-4 kg.m2. The momentum calculation shows that 80% of the momentum is in the rotating ballscrew alone, 12.5% in the armature of the servo
    and the remaining 7.5% in the linear axis mass. So despite the axis weighing 150kg its still a minor contributor to the momentum. Thus if I decided to add a whole bunch of mass to the axis, say a trunnion table
    and fifth axis, the extra mass is STILL a minor to modest contributor to the overall momentum.

    Give us the details of the axis and we'll see just how important, or not, the mass is.

    Craig
    Thanks Graig. My inexperience probably makes me want to be over cautious. Its probably worth saying the nonferrous metals would be more of a bonus than anything essential, speed would not be a real concert cutting them either.

    The z axis details are SFU1605 ballscrew so 16mm diameter 5mm pitch, 30cm long, it will use a 4nm nema 23 motor. The spindle is a chinese 2.2k one.

  11. #11
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    Re: Any thought on my Z axis Design (first build) Steel vs Aluminium Plate?

    Hi Thunza - Make the flanges thick and the Z plate thick - as thick as you can. You have other hurdles to clear. Are you welding this? Bolting? brazing etc. There maybe a commercial channel in alum or steel? Mine are laser cut and bent. You also have to consider if you are going to machine the rail foundations. Then there is access to bolts on the saddle cars, you have many more issues to resolve in this area. You have the drive nut under the spindle clamp. This will be a pain to maintain as you will have to remove the spindle to check or undo the drive bolts etc. Don't bury any bolt heads!! Is the spindle 2.2kW if so consider downsizing it to 1.5kW or even 800W. A 2.2kW is a big beefy spindle and not needed for wood or aluminium. The size of thr 2.2 will drive many areas to be oversize. If your trying to keep things compact go with aq 800W water cooled spindle... The only reason to go with a 2.2kW spindle is to get a ER20 collet so you can run 1/2" tools. Thats serious cutting territory and your machine won't match its capabilities... Peter

    edit - ductility and material stiffness are not related. Some of the stiffest materials we have are ceramics and they fail at <1% strain. Metals are ductile because they have grains that are held together by friction just like lego blocks so they can slip along each other when stretched. Crystalline structures have no slip are very strong and stiff.

    speed of machine cutting metals is nearly irrelevant. The machine still has to be stiff enough cutting slow or fast. maximise the geometry of every part, chase stiffness with every part. What is the work envelope you are aiming at?

  12. #12
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    Re: Any thought on my Z axis Design (first build) Steel vs Aluminium Plate?

    Hi,
    OK.

    The first moment of inertia of the ballscrew is:
    Jscrew= pi x r2 x l x rho x 1/2 x r2 where r=radius of the screw, l is the length and rho is the density of steel
    =0.51 x 10-4kg.m2

    The first moment of inertia of a 23/24 size stepper is approx:
    Jstepper=0.7 x 10-4kg.m2

    The first moment of inertia of the load, assuming 20kg:
    Jload= (m x p2)/4.pi2 where m is the axis mass and p is the pitch
    =0.13 x 10-4 kg.m2

    So the momentum balance is:
    38% is the ballscrew
    52% is the armature of the stepper
    10% is the linear axis mass

    The important thing here is that the mass of the axis is a small proportion (10%) of the momentum thus making it a bit heavier or lighter has a very small effect
    overall. Given that is the case then making a plate, while being somewhat heavier is not a disadvantage or least enough to outweigh the advantage of simplicity in terms
    of manufacture and cost of materials.

    Craig

  13. #13

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    Re: Any thought on my Z axis Design (first build) Steel vs Aluminium Plate?

    Quote Originally Posted by peteeng View Post
    Hi Thunza - Make the flanges thick and the Z plate thick - as thick as you can. You have other hurdles to clear. Are you welding this? Bolting? brazing etc. There maybe a commercial channel in alum or steel? Mine are laser cut and bent. You also have to consider if you are going to machine the rail foundations. Then there is access to bolts on the saddle cars, you have many more issues to resolve in this area. You have the drive nut under the spindle clamp. This will be a pain to maintain as you will have to remove the spindle to check or undo the drive bolts etc. Don't bury any bolt heads!! Is the spindle 2.2kW if so consider downsizing it to 1.5kW or even 800W. A 2.2kW is a big beefy spindle and not needed for wood or aluminium. The size of thr 2.2 will drive many areas to be oversize. If your trying to keep things compact go with aq 800W water cooled spindle... The only reason to go with a 2.2kW spindle is to get a ER20 collet so you can run 1/2" tools. Thats serious cutting territory and your machine won't match its capabilities... Peter

    edit - ductility and material stiffness are not related. Some of the stiffest materials we have are ceramics and they fail at <1% strain. Metals are ductile because they have grains that are held together by friction just like lego blocks so they can slip along each other when stretched. Crystalline structures have no slip are very strong and stiff.

    speed of machine cutting metals is nearly irrelevant. The machine still has to be stiff enough cutting slow or fast. maximise the geometry of every part, chase stiffness with every part. What is the work envelope you are aiming at?

    Hi Peter thanks for sharing your knowledge. My initial intention is to use it for musical instruments like Harps, perhaps the odd bit of furniture and artistic pieces too. Part of my reason for going down this route is by enclosing the cnc and using it instead of other equipment I hope can reduce my exposure to dust which had been giving me some trouble because of allergies, I work in an old garage without a main door, keeping my extraction ducted to outside.

    You are probably right about the spindle size, my routers which are lower KW are capable, I think I liked the idea of a ER20 collect for having larger diameter tool bits, enabling deeper cuts with more rigidity. Apart from carving 3d shapes I can't see the need to cut through more than 5cm. Spindles aren't too expensive and I could sell the old one so swapping is feasible.

    As far as frame goes I plan use box steel, bolting combined with metalized epoxy, perhaps roughly following this sort of method. I have looked into buying a stick welder but having zero experience and hearing that they can wrap the steel. I would only be able to power a welder capable for doing 3mm corner joints too, with bolting I can go thicker.

    Machine frame - MadVac CNC

    I'd like to level the top rails with epoxy probably followed up with scraping.

  14. #14

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    Re: Any thought on my Z axis Design (first build) Steel vs Aluminium Plate?

    Quote Originally Posted by joeavaerage View Post
    Hi,
    OK.

    The first moment of inertia of the ballscrew is:
    Jscrew= pi x r2 x l x rho x 1/2 x r2 where r=radius of the screw, l is the length and rho is the density of steel
    =0.51 x 10-4kg.m2

    The first moment of inertia of a 23/24 size stepper is approx:
    Jstepper=0.7 x 10-4kg.m2

    The first moment of inertia of the load, assuming 20kg:
    Jload= (m x p2)/4.pi2 where m is the axis mass and p is the pitch
    =0.13 x 10-4 kg.m2

    So the momentum balance is:
    38% is the ballscrew
    52% is the armature of the stepper
    10% is the linear axis mass

    The important thing here is that the mass of the axis is a small proportion (10%) of the momentum thus making it a bit heavier or lighter has a very small effect
    overall. Given that is the case then making a plate, while being somewhat heavier is not a disadvantage or least enough to outweigh the advantage of simplicity in terms
    of manufacture and cost of materials.

    Craig
    Thanks that's interesting to see. I don't think I have enough knowledge to understand confidently but I think I can see your paint point about weight being a rather small element.

  15. #15
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    Re: Any thought on my Z axis Design (first build) Steel vs Aluminium Plate?

    Hi,

    but I think I can see your paint point about weight being a rather small element.
    Yes, that is the most important take away.

    As recently as a couple of years ago I'd never bothered to do the detailed calculation and like you imagined that the axis mass would
    dominate the momentum and acceleration equations. Only once I started to do the calculations myself did I realise how wrong I was.
    Hiwin and Yaskawa publish the same equations albeit in differing and confusing units, but my calculations agree exactly with the principles published.

    Again the important point it to be aware of those elements that affect machine performance the most and recognize that other elements, in this case the Z axis mass,
    make a small contribution. Thus while you can't be cavalier about the axis mass neither is it a critical parameter.

    Craig

  16. #16
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    Re: Any thought on my Z axis Design (first build) Steel vs Aluminium Plate?

    Hi Thunza - For your application I'd stay away from metal nearly all together. Frankie is mainly F17 formply which is stiff and damp. I have not run it yet but it feels very stiff and I'm sure its strong from building a machine from MDF and having tested that thoroughly for a couple of years. Made lots of guitar parts for people. The form ply is significantly better.. Look up the Frankenrouter thread... Since you are heading for an enclosure I would integrate the "walls" as the enclosure sides and extend them up to create the roof. I would not use the 2.2kW spindle. If tool size is an issue use a 1.5kW with an ER16 collet. This will give you 10mm tools which can be quite long. The size of the 2.2kW will flow into the rest of the machine making it bigger than needed. If its bigger, then it won't be as stiff and you will chase your tail... The size of the Z axis parts (like the spindle clamp) ultimately drives the machine footprint. You will be surprised as you flesh this out how the machine will grow and grow.

    If you use metal frames bolt them together and if the connection is permanent, then epoxy the joint. This will be 95% as stiff as welded and damper. This way the joint can be adjusted until correct then set with the epoxy. Good luck with epoxy levelling and scraping I would not go near that that can of worms and wombat holes... For instance in steel structures design, the welded damping coefficient is zero and the bolted connection is 2% which is quite damp...

    Hi Craig - You use the word "momentum" to describe "inertia" please stop, this is annoying lay use of momentum is to describe something that wants to carry on moving. Engineering momentum is mass x velocity. Momentum is conserved in a system and we use it to calculate various things like collison velocities. Momentum - Wikipedia

    What you are describing is "inertia" and inertia is the objects RESISTANCE to motion. You do use this correctly in spots. So the inertia of the screws and masses have to be overcome by the motor. Geometric inertia is the members resistance to bending for instance... I have sleep inertia sometimes and don't want to get out of bed...Sleep inertia - Wikipedia Thunza keep at it... Peter

  17. #17
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    Re: Any thought on my Z axis Design (first build) Steel vs Aluminium Plate?

    Hi,

    You use the word "momentum" to describe "inertia" please stop, this is annoying
    Don't be a nit picking jerk.....I now your Aussie but that's no excuse for this crap. I too did a degree in Engineering and that's based on a Physics education that precedes that for some years, if there is anyone
    who objects to the terms I use its you....then get used to it.

    Craig

  18. #18

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    Re: Any thought on my Z axis Design (first build) Steel vs Aluminium Plate?

    Thanks again Peter.

    I'm still looking into the epoxy leveling but can't see much alternative without paying a lot of have the rail supports machined.

    I'll definitely consider your recommendation to downsize. I don't think i'll need a huge layout in terms of width, so i might be okay with as little as 40cm. It's good to hear you are confident about the screw and epoxy joining method. There do seem to be some 1500w spindles with a ER20 collect, I noticed if theyre watercooled they are only about .5kg less weight though

  19. #19

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    Re: Any thought on my Z axis Design (first build) Steel vs Aluminium Plate?

    Quote Originally Posted by joeavaerage View Post
    Hi,



    Yes, that is the most important take away.

    As recently as a couple of years ago I'd never bothered to do the detailed calculation and like you imagined that the axis mass would
    dominate the momentum and acceleration equations. Only once I started to do the calculations myself did I realise how wrong I was.
    Hiwin and Yaskawa publish the same equations albeit in differing and confusing units, but my calculations agree exactly with the principles published.

    Again the important point it to be aware of those elements that affect machine performance the most and recognize that other elements, in this case the Z axis mass,
    make a small contribution. Thus while you can't be cavalier about the axis mass neither is it a critical parameter.

    Craig
    Very helpful, thanks you! I guess even if the stepper can handle it I would still need to make the Y axis robust enough not to sag.

  20. #20
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    Re: Any thought on my Z axis Design (first build) Steel vs Aluminium Plate?

    Hi,

    I would still need to make the Y axis robust enough not to sag.
    If the Y axis, or any axis, sags under its own weight then its grossly too flexible to be considered anything like rigid enough for CNC purposes.

    Craig

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